Every year the containers on my deck multiply like rabbits. One pot of basil becomes three, a cherry tomato fills a pot so big I can barely move it, and let’s not get started on the petunias. While it’s great fun to grow plants up close, container gardening can cost a small fortune in potting soil. Why let it go to waste? With thoughtful handling, you can reuse potting soil in next year’s containers, or use it to solve other gardening problems.
The first step is to let used potting soil dry out, either in pots or dumped into a wheelbarrow or onto a tarp. I favor the dump method for two reasons. In my climate pots must be stored empty and dry to keep them from cracking in winter, and it’s easy to comb through a mountain of loose soil to remove stringy roots. Old potting soil need not be bone dry when you store it, but too much moisture can create cushy conditions for unwanted moldy microbes. Dry soil weighs less, too.
Storing Potting Soil
I like to store potting soil used to grow edibles separate from the stuff that supported flowers, which helps limits disease carryover from one year to the next the same way rotations work in the vegetable garden. The soil used for flowers this year is fair game for edibles next year, and vice versa. Any storage containers that will keep the soil dry will work, including bins, small garbage cans, or heavy-duty plastic bags.
Exposure to freezing temperatures is good for stored potting soil, because it will make life difficult or impossible for any insects that may be present as adults, pupae or eggs. When stored under warm conditions, used potting soil may give rise to mysterious hatches.
Good Uses for Old Potting Soil
Much of my old potting soil is not used for potting up plants. Though it may lack nutrients, old potting soil still contains nuggets of perlite, threads of humus, and very few weed seeds. This makes it an ideal material for covering newly planted carrots, beets and other slow-sprouting seeds. A topdressing of moisture-holding potting soil enhances germination of the seeded crop, with fewer weeds competing for space.
Used potting soil also comes in handy when moles, dogs or other critters create holes in the lawn that need to be filled and patched. Grass seed that is covered with a thin layer of potting soil usually comes up strong, with few unwanted weeds.
You also can use old potting soil to pot up giveaway plants. Sharing divisions taken from asters, bee balm, daylilies and other vigorous perennials costs nothing when you drop them in a cracked plastic pot and snug them in with used potting soil.
Rejuvenating Used Potting Soil
Many gardeners simply mix used potting soil with new material, using about half of each, with a few handfuls of organic fertilizer added to boost plant nutrition. Or, you can place the old potting soil in the bottoms of very large containers, and fill the upper parts with a fresh mix.
This simple practice works well with soil that hosted healthy plants, but my humid climate is rife with blights and mildews, so I take the extra step of heat-treating potting soil that was used to grow edibles. Only 30 minutes of exposure to temperatures above 120°F (49°C) will kill most disease pathogens, but you need not stink up your house by using your oven. Instead, put a few gallons of used potting soil in a black plastic bag, and place the bag inside a translucent storage bin set in full sun on a bright day. A parked car with the windows rolled up makes a good solarization chamber, too. Once used potting soil has been heated and cooled, it’s ready to add to any type of new mixture you want to create.
I still use a fresh bag of seed-starting mix for starting seeds, but by recovering and storing much of the soil from outdoor containers, I make far fewer trips to the garden center for store-bought dirt.